Conventional thermochromic materials as described in, for example, U.S. Pat. No. 4,028,118 (which corresponds to British Pat. No. 1,405,701, French Patent No. 7,319,876 and German Patent Application (OLS) 2,327,723), are not suitable for outdoor applications since they contain, as a colorforming component, an electron-donating, chromatic organic compound which is generally of low light-fastness and, therefore, they are readily subject to fading on exposure to sunlight.